N

Figure 2.8 Nicotinamide adenine dinucleotide NAD+ (or NADF" with an extra phosphate on C-2 of ribose) oxidized and reduced forms. The sphere in the right-hand structure represents the remainder of the molecule

The reaction of a general alcohol with NAD+ is illustrated in Figure 2.9. Note that when the alcohol is held on the enzyme surface it is possible to distinguish between the pro-chiral hydrogen atoms (for an explanation see the section on chirality at the end of this chapter). We know that the pro-R hydrogen atom is removed, and becomes attached to the upper side (as drawn) of the reduced nicotinamide molecule. When NADH is used for reduction, the same hydrogen, from the upper side of the molecule is transferred to the back of the carbonyl group (as drawn in Figure 2.9).

"NH2_fVWNH2

alcohol dehydrogenase enzyme

The overall reaction is: RCH2OH + NAD+ CT , R-CHO + NADH + H+ CI"

Figure 2.9 Oxidation of an alcohol with NAD+, and its reverse reaction, reduction of an aldehyde or ketone. B: represents some basic group on the enzyme. The subscripts R and S are a means of distinguishing between the two hydrogen atoms of the methylene group, see the section on chirality at the end of this chapter

This has been established by replacing either the pro-S or the pro-R hydrogen by deuterium and seeing whether the deuterium is retained by acetaldehyde or is taken up by NADH.

NAD+ is used in metabolism or catabolism, for example, turning sugars into energy, NADP+ is used for anabolism or building up chemicals. Nature, in this way, keeps the two kinds of process separated.